JP7695041B2 - Negative electrode and secondary battery including the same - Google Patents

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of priority based on Korean Patent Application No. 10-2020-0048940, filed on April 22, 2020, and all contents disclosed in the documents of the Korean patent application are incorporated herein by reference.

The present invention relates to an anode including a negative electrode current collector, a first negative electrode active material layer disposed on the negative electrode current collector, and a second negative electrode active material layer disposed on the first negative electrode active material layer, the second negative electrode active material layer including a second negative electrode active material and a second conductive material, the second negative electrode active material including a silicon-based active material, the silicon-based active material including SiO _x (0≦X<2), the second conductive material including a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded in a line, and the carbon nanotube structure is included in the second negative electrode active material layer in an amount of 0.01 wt % to 1.0 wt %, and a secondary battery including the same.

Recently, with technological development and increased demand for mobile devices, the demand for batteries as an energy source has increased dramatically, and as a result, various researches are being conducted on batteries that can meet various demands. In particular, active research is being conducted on lithium secondary batteries that have high energy density as well as excellent life and cycle characteristics as a power source for such devices.

A lithium secondary battery is a battery that includes a positive electrode assembly having a positive electrode active material capable of inserting/desorbing lithium ions, a negative electrode having a negative electrode active material capable of inserting/desorbing lithium ions, and a microporous separator between the positive electrode and the negative electrode, and a non-aqueous electrolyte containing lithium ions.

On the other hand, the negative electrode active material alone cannot ensure the conductivity of the negative electrode, which can lead to a problem of the battery's resistance being too high. For this reason, the negative electrode usually further contains a conductive material. Conventionally, dot-shaped conductive materials such as carbon black have been mainly used, and linear conductive materials such as carbon nanotubes and carbon nanofibers have also been used to further improve the conductivity and thus the capacity of the battery.

Single-walled carbon nanotubes are one of the linear conductive materials, and their elongated shape improves the conductivity in the negative electrode active material layer. For this reason, in the past, negative electrode slurry was produced using a dispersion liquid in which the single-walled carbon nanotubes were completely dispersed, and then the negative electrode active material layer was produced using the negative electrode slurry.

However, when the battery is repeatedly charged and discharged, the negative electrode active material repeatedly expands and contracts in volume, causing the single-walled carbon nanotubes to break, making it difficult to maintain the conductive network in the negative electrode active material layer. In particular, when a silicon-based active material is used as the negative electrode active material to improve the capacity of the battery, the volume of the silicon-based active material expands too much due to the charging and discharging of the battery, causing the single-walled carbon nanotubes to break more severely. This causes the conductive network to be cut off or reduced, which reduces the life characteristics of the battery. In addition, since the single-walled carbon nanotubes are present by wrapping the surface of the silicon-based active material, they cannot smoothly perform their role of conductively connecting adjacent negative electrode active materials to each other.

On the other hand, when carbon nanotubes are used as a conductive material, a carbon nanotube dispersion with a low solid content must be used in order to uniformly distribute the carbon nanotubes in the negative electrode active material layer. However, when carbon nanotubes with a low solid content are used, a migration phenomenon occurs in which the binder, which has a relatively low density compared to the negative electrode active material, and the conductive material easily migrate to the upper layer of the negative electrode active material layer (away from the current collector) when the negative electrode is dried, resulting in a significant decrease in the negative electrode adhesive strength and electrical conductivity.

Therefore, the present invention introduces an anode that can connect a conductive network even with large volume changes in the anode active material, thereby minimizing problems caused by the binder tilt phenomenon.

The problem that the present invention aims to solve is to provide a negative electrode that can improve input/output characteristics and life characteristics by minimizing problems caused by the binder tilt phenomenon while smoothly maintaining a conductive network.

Another problem that the present invention aims to solve is to provide a secondary battery that includes the negative electrode.

According to one embodiment of the present invention, there is provided a negative electrode including a negative electrode current collector, a first negative electrode active material layer disposed on the negative electrode current collector, and a second negative electrode active material layer disposed on the first negative electrode active material layer, the second negative electrode active material layer including a second negative electrode active material and a second conductive material, the second negative electrode active material including a silicon-based active material, the silicon-based active material including SiO _x (0≦X<2), the second conductive material including a carbon nanotube structure including 2 to 5,000 single-walled carbon nanotube units bonded in a row, and the carbon nanotube structure is included in the second negative electrode active material layer in an amount of 0.01 wt % to 1.0 wt %.

According to another embodiment of the present invention, a secondary battery including the negative electrode is provided.

In the negative electrode according to the present invention, the second negative active material layer includes a long rope-shaped carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are aligned and bonded together during rolling, so that the carbon nanotube structure can connect the second negative active material to each other and strongly hold the SiO _x (0≦X<2) of the second negative active material layer even when the volume of the SiO _x (0≦X<2) of the second negative active material layer changes significantly, and the SiO x (0≦X<2) can be prevented from being damaged (e.g., cracked). In addition, since the negative electrode has the first negative active material layer and the second negative active material layer sequentially arranged as respective slurries, the aforementioned inclination phenomenon of the binder and the conductive material can be minimized. Furthermore, since the second negative active material layer includes a carbon nanotube structure, the adhesive force between the first negative active material layer and the second negative active material layer can be strengthened. As a result, the input/output characteristics and life characteristics of the battery can be improved.

4 is a photograph showing an analysis of binder distribution in the negative electrode of Comparative Example 1 and the negative electrode of Example 1. 4 is a SEM photograph of a second negative electrode active material layer of the negative electrode of Example 1. 13 is a SEM photograph of a second negative electrode active material layer of the negative electrode of Example 2. 13 is a SEM photograph of a second negative electrode active material layer of the negative electrode of Comparative Example 3. 4 is a SEM photograph of a second negative electrode active material layer of the negative electrode of Example 1. 13 is a SEM photograph of a second negative electrode active material layer of the negative electrode of Comparative Example 2.

The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary and dictionary meanings, but should be interpreted in a way that is consistent with the technical ideas of the present invention, based on the principle that the inventor himself/herself can appropriately define the concept of the term in order to best describe the invention.

The terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the present invention. Singular expressions may include plural expressions unless the context clearly indicates otherwise.

In this specification, the terms "including," "comprise," "have," and the like are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, and should be understood as not precluding the presence or additional possibility of one or more other features, numbers, steps, components, or combinations thereof.

In this specification, "%" means % by weight unless otherwise specified.

In this specification, "specific surface area" is measured by the BET method, and specifically, can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77K) using a BELSORP-mini II from BEL Japan.

In this specification, the average particle size ( _D50 ) may be defined as a particle size corresponding to 50% of the cumulative volume in a particle size distribution curve of the particles. The average particle size ( _D50 ) may be measured, for example, by a laser diffraction method. The laser diffraction method generally allows measurement of particle sizes from the submicron range to several mm, and provides results with high reproducibility and high resolution.

In the present invention, a single-wall carbon nanotube unit refers to a tubular unit having one wall made of carbon atoms, and a multi-wall carbon nanotube unit refers to a tubular unit having multiple walls made of carbon atoms in one tube.

The present invention will be described in detail below.

A negative electrode according to the present invention includes a negative electrode current collector, a first negative electrode active material layer disposed on the negative electrode current collector, and a second negative electrode active material layer disposed on the first negative electrode active material layer, the second negative electrode active material layer including a second negative electrode active material and a second conductive material, the second negative electrode active material including a silicon-based active material, the silicon-based active material including SiO _x (0≦X<2), the second conductive material including a carbon nanotube structure including 2 to 5,000 single-walled carbon nanotube units bonded in a row, and the carbon nanotube structure may be included in the second negative electrode active material layer in an amount of 0.01 wt % to 1.0 wt %.

The negative electrode current collector is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, the negative electrode current collector may be made of copper, stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like. Specifically, a transition metal that easily adsorbs carbon, such as copper or nickel, may be used as the negative electrode current collector.

The negative electrode may include a negative electrode active material layer. The negative electrode active material layer may be disposed on one or both sides of the negative electrode current collector.

The negative electrode active material layer may include a first negative electrode active material layer and a second negative electrode active material layer. The first negative electrode active material layer may be disposed on the negative electrode current collector, specifically, may be in contact with the negative electrode current collector. The second negative electrode active material layer may be disposed on the first negative electrode active material layer, and the first negative electrode active material layer may be disposed between the second negative electrode active material layer and the negative electrode current collector.

In general, when carbon nanotubes are used as a conductive material, a carbon nanotube dispersion liquid with a low solid content must be used to uniformly arrange the carbon nanotubes in the negative electrode active material layer. However, when a carbon nanotube dispersion liquid with a low solid content is used, when the negative electrode slurry is dried, the binder and conductive material, which have a relatively low density compared to the negative electrode active material, tend to easily tilt (migration) toward the upper layer of the negative electrode active material layer (the side farther from the negative electrode current collector and closer to the surface), resulting in a significant decrease in the negative electrode adhesive strength and electrical conductivity. On the other hand, the negative electrode of the present invention has a first negative electrode active material layer and a second negative electrode active material layer arranged in sequence as respective slurries, so that the tilt phenomenon of the binder and conductive material can be minimized. As a result, the input/output characteristics and life characteristics of the battery can be improved.

(1) First Negative Electrode Active Material Layer The first negative electrode active material layer may include a first negative electrode active material.

The first negative electrode active material may be a negative electrode active material commonly used in the technical field, and the type is not particularly limited.

Specifically, the first negative electrode active material may include at least one of a carbon-based active material and a silicon-based active material, and the carbon-based active material particles may be one or more selected from the group consisting of artificial graphite, natural graphite, graphitized carbon fiber, and graphitized mesocarbon microbeads, and in particular, when artificial graphite is used, the rate characteristics can be improved. The silicon-based active material may be one or more selected from the group consisting of SiO _x (0≦X<2), Si-C composite, and Si-Y alloy (wherein Y is an element selected from the group consisting of alkali metals, alkaline earth metals, transition metals, Group 13 elements, Group 14 elements, rare earth elements, and combinations thereof), and in particular, when SiO _x (0≦X<2) is used, a high capacity of the battery can be derived. More specifically, the first negative electrode active material may be a carbon-based active material.

The first negative electrode active material may be contained in the first negative electrode active material layer in an amount of 70% by weight to 99.5% by weight, preferably 80% by weight to 99% by weight. When the content of the first negative electrode active material satisfies this range, the energy density of the negative electrode is improved, the negative electrode adhesion is increased, and the electrical conductivity in the negative electrode is improved.

The first negative electrode active material layer may further include a first conductive material.

The first conductive material may include at least one selected from the group consisting of a carbon nanotube structure, a multi-walled carbon nanotube unit, graphene, and carbon black. The carbon nanotube structure will be described in detail below.

The first conductive material may be included in the first negative electrode active material layer in an amount of 0.01 wt % to 2.0 wt %, specifically 0.01 wt % to 1.5 wt %, more specifically 0.05 wt % to 1.0 wt %. If this range is met, the adhesion and electrical conductivity of the negative electrode can be significantly improved even with only a small amount of the first conductive material, and a battery with excellent input/output characteristics and life characteristics can be achieved.

The thickness of the first negative electrode active material layer may be 1 μm to 100 μm, specifically 5 μm to 90 μm, more specifically 10 μm to 80 μm. When this range is satisfied, the aforementioned conductive material and binder tilt phenomenon can be minimized. As a result, the adhesive strength and electrical conductivity of the negative electrode are significantly improved, and the input/output characteristics and life characteristics of the battery can be improved.

(2) Second Negative Electrode Active Material Layer The second negative electrode active material layer may include a second negative electrode active material and a second conductive material.

The second negative electrode active material may include a silicon-based active material.

The silicon-based active material may include _SiOx (0 < x < 2). The _SiOx (0 < x < 2) may be SiO. Since the second negative electrode active material includes _SiOx (0 < x < 2), the capacity of the battery can be improved. In particular, since the second negative electrode active material layer includes _SiOx (0 < x < 2) instead of the first negative electrode active material layer, the durability of the negative electrode is improved and the impregnation of the electrolyte can be improved. More specifically, at the interface between the negative electrode collector and the negative electrode active material layer, which has the weakest binding force in the negative electrode, there is a problem that the negative electrode active material is easily detached from the negative electrode due to contraction and expansion of the negative electrode active material during charging and discharging of the battery, and when _SiOx (0 < x < 2) is located near the negative electrode collector, the detachment phenomenon is accelerated. This reduces the durability of the negative electrode, and the capacity and life characteristics of the battery are deteriorated.

Meanwhile, when the negative electrode is rolled during the manufacturing process, the density near the surface of the negative electrode becomes very high, which reduces the impregnation of the electrolyte. However, when _SiOx (0≦X<2) is located near the surface of the negative electrode, the volume expansion of _SiOx (0≦X<2) during initial charging of the battery reduces the density of the negative electrode to an appropriate level, thereby improving the impregnation of the electrolyte.

The silicon-based active material may further include a carbon coating layer formed on the SiO _x (0≦X<2). The carbon coating layer may be disposed on the SiO _x (0≦X<2). The carbon coating layer serves to improve the electrical conductivity of the SiO _x (0≦X<2) and to suppress excessive volume expansion of the SiO _x (0≦X<2).

The carbon coating layer may include at least one of amorphous carbon and crystalline carbon.

The crystalline carbon can further improve the electrical conductivity of the negative electrode active material. The crystalline carbon may include at least one selected from the group consisting of fullerene, carbon nanotube, and graphene.

The amorphous carbon can appropriately maintain the strength of the coating layer and suppress the expansion of the natural graphite. The amorphous carbon can be at least one carbide selected from the group consisting of tar, pitch, and other organic substances, or a carbon-based material formed by using a hydrocarbon as a source for chemical vapor deposition.

The carbonized organic matter may be a carbonized organic matter selected from carbonized sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldohexose, or ketohexose, and combinations thereof.

The hydrocarbon may be a substituted or unsubstituted aliphatic or alicyclic hydrocarbon, or a substituted or unsubstituted aromatic hydrocarbon. The aliphatic or alicyclic hydrocarbon of the substituted or unsubstituted aliphatic or alicyclic hydrocarbon may be methane, ethane, ethylene, acetylene, propane, butane, butene, pentene, isobutene, or hexene. The aromatic hydrocarbon of the substituted or unsubstituted aromatic hydrocarbon may be benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumarone, pyridine, anthracene, or phenanthrene.

The average particle size (D ₅₀ ) of the silicon-based active material may be 0.1 μm to 20 μm, specifically, 1 μm to 10 μm. When this range is satisfied, a side reaction between the SiO _x (0≦X<2) and the electrolyte can be suppressed, and the reaction of forming lithium silicate from the SiO _x (0≦X<2) can be controlled, thereby preventing a decrease in initial efficiency and maximizing the initial capacity of the battery.

The second negative electrode active material may further include a carbon-based active material. The carbon-based active material may include at least one selected from the group consisting of artificial graphite, natural graphite, and graphitized mesocarbon microbeads. Specifically, the carbon-based active material is preferably artificial graphite, but is not limited thereto, in that it can effectively control the volume expansion of the negative electrode while maintaining an electrical network together with the carbon nanotube structure described below.

The weight ratio of the silicon-based active material to the carbon-based active material may be 0.5:99.5 to 20:80, specifically 1:99 to 10:90. When this range is satisfied, excessive volume expansion of the second negative electrode active material can be suppressed, and the capacity of the battery can be improved.

The second negative electrode active material may be contained in the second negative electrode active material layer at 90% by weight to 99% by weight, specifically, at 95% by weight to 99% by weight. When this range is satisfied, the energy density of the negative electrode can be maintained high, and the electrical conductivity and adhesive strength of the negative electrode can be improved.

The second conductive material may further include a carbon nanotube structure.

The carbon nanotube structure may include a plurality of single-walled carbon nanotube units. Specifically, the carbon nanotube structure may be a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are aligned and bonded to each other. More specifically, in consideration of the durability and conductive network of the second negative electrode active material layer, the carbon nanotube structure may be a carbon nanotube structure in which 2 to 4,500, preferably 2 to 4,000, more preferably 2 to 200 single-walled carbon nanotube units are aligned and bonded to each other. In consideration of improving the dispersibility of the carbon nanotube structure and the durability of the negative electrode, the carbon nanotube structure may be a carbon nanotube structure in which 2 to 50 single-walled carbon nanotube units are aligned and bonded to each other.

In the carbon nanotube structure, the single-walled carbon nanotube units may be arranged side by side and bonded together (the long axes of the units are bonded together in parallel to form a flexible bundled cylindrical structure) to form the carbon nanotube structure. In the second negative electrode active material layer, the carbon nanotube structures may be connected to each other to form a network structure.

Conventional electrodes including carbon nanotubes are generally prepared by dispersing bundle-type or entangled-type carbon nanotubes (single-walled carbon nanotube units or multi-walled carbon nanotube units attached or entangled with each other) in a dispersion medium to prepare a conductive material dispersion, and then using the conductive material dispersion. In this case, the carbon nanotubes are completely dispersed in the conventional conductive material dispersion, and exist as a conductive material dispersion in which single-stranded carbon nanotube units are dispersed. In the conventional conductive material dispersion, the carbon nanotube units are easily cut due to an excessive dispersion process, and have a short length compared to the initial length. In addition, the carbon nanotube units may be easily cut during the rolling process of the negative electrode, and there is an additional problem that the carbon nanotube units (especially single-walled carbon nanotube units) are cut due to excessive volume change of the silicon-based active material during the operation of the battery. As a result, the conductivity of the negative electrode is reduced, and the life characteristics of the battery are reduced. Furthermore, in the case of multi-walled carbon nanotube units, the structure has high defects due to the mechanism of node growth (nodes exist due to defects that occur during the growth process rather than smooth lines). Therefore, during the dispersion process, the multi-walled carbon nanotube units are more easily cut, and the short cut multi-walled carbon nanotube units are more likely to aggregate with each other due to π-π interaction (π-π stacking) between the carbons of the units. As a result, they are less likely to be uniformly dispersed in the negative electrode slurry.

In contrast, in the case of the carbon nanotube structure included in the second negative electrode active material layer of the present invention, since it has a rope-like shape in which 2 to 5,000 single-walled carbon nanotube units that maintain high crystallinity relatively without structural defects are arranged side by side and connected to each other, it can smoothly maintain its length without being cut even when the volume of the second negative electrode active material changes, so that the conductivity of the negative electrode can be maintained even during continuous charging and discharging of the battery. In addition, due to the high electrical conductivity of the single-walled carbon nanotube units with high crystallinity, the conductivity of the negative electrode can be increased, the resistance of the negative electrode can be reduced, and the input/output characteristics and life characteristics of the battery can be significantly improved. In addition, in the second negative electrode active material layer that is directly subjected to pressure during rolling, the carbon nanotube structures can be connected to each other to form a network structure, so that damage to the second negative electrode active material (e.g., cracks and other cracking phenomena) can be suppressed. In addition, even if a crack occurs in the second negative electrode active material, the carbon nanotube structure crosses the crack to connect the second negative electrode active material, so that the conductive network can be maintained. Furthermore, since the carbon nanotube structure is not easily cut and can maintain its long shape, the conductive network can be strengthened throughout the second negative electrode active material layer. Also, detachment of the second negative electrode active material is suppressed, and the negative electrode adhesive strength can be significantly improved.

In addition, since the carbon nanotube structure is included in the second negative electrode active material layer, the adhesive strength between the first negative electrode active material layer and the second negative electrode active material layer can be significantly improved. The carbon nanotube structure has a long rope shape formed by the horizontal bonding of the single-walled carbon nanotube units inside, and therefore the second negative electrode active material can be well connected to each other by van der Waals forces, thereby forming a strong negative electrode. Furthermore, the carbon nanotube structure and the surface of the carbon-based active material of the first negative electrode active material layer can be more tightly bonded due to π-π bonds (stacking) that occur between the same type of carbon, so that the adhesive strength between the first negative electrode active material layer and the second negative electrode active material layer can be further strengthened.

In the carbon nanotube structure, the average diameter of the single-walled carbon nanotube units may be 0.5 nm to 10 nm, specifically, 1 nm to 9 nm. If the average diameter is satisfied, the conductivity in the negative electrode can be maximized even with a very small amount of conductive material. The average diameter corresponds to the average value of the top 100 single-walled carbon nanotube units with the largest diameter and the bottom 100 single-walled carbon nanotube units with the smallest diameter when the manufactured negative electrode is observed through a TEM.

In the carbon nanotube structure, the average length of the single-walled carbon nanotube units may be 1 μm to 100 μm, specifically, 5 μm to 50 μm. When the average length is satisfied, a long conductive path for conductive connection between the second negative electrode active materials may be formed, and a unique network structure may be formed, so that even with a very small amount of conductive material, the conductivity in the negative electrode can be maximized. The average length corresponds to the average value of the top 100 single-walled carbon nanotube units and the bottom 100 single-walled carbon nanotube units when the manufactured negative electrode is observed through a TEM.

The specific surface area of the single-walled carbon nanotube unit may be 500 m ² /g to 1,000 ^{m 2} /g, and more specifically, 600 m ² /g to 800 m ² /g. If the range is satisfied, the wide specific surface area can smoothly secure a conductive path in the negative electrode, and therefore, even with a very small amount of conductive material, the conductivity in the negative electrode can be maximized. Specifically, the specific surface area of the single-walled carbon nanotube unit can be calculated from the nitrogen gas adsorption amount at liquid nitrogen temperature (77K) using BELSORP-mini II from BEL Japan.

The average diameter of the carbon nanotube structures may be 2 nm to 500 nm, specifically 5 nm to 200 nm, and more specifically 5 nm to 50 nm. When this range is satisfied, it is effective for forming a conductive network structure, is advantageous for connecting the second negative electrode active materials, and can realize excellent electrical conductivity. The average length corresponds to the average value of the diameters of the top 100 carbon nanotube structures with the largest diameters and the bottom 100 carbon nanotube structures when the manufactured negative electrode is observed through a SEM.

The average length of the carbon nanotube structures may be 1 μm to 500 μm, specifically 1 μm to 100 μm, and more specifically 2 μm to 50 μm. When this range is satisfied, it is effective for forming a conductive network, is advantageous for connecting the second negative electrode active materials, and can realize excellent electrical conductivity. The average length corresponds to the average value of the lengths of the top 100 carbon nanotube structures and the bottom 100 carbon nanotube structures when the manufactured negative electrode is observed through a SEM.

The carbon nanotube structure may be included in the second negative electrode active material layer at 0.01 wt % to 1.0 wt %, specifically at 0.01 wt % to 0.5 wt %, and more specifically at 0.01 wt % to 0.2 wt %. When this range is satisfied, the conductive path of the second negative electrode active material layer is secured, and the negative electrode resistance can be maintained at a low level while improving the battery life characteristics. When the bundled carbon nanotubes are completely dispersed (a typical dispersion method in which the carbon nanotube units are dispersed as far apart as possible) during the preparation of the conductive material dispersion liquid, the carbon nanotube structure does not occur or, even if it does occur unintentionally, it occurs in a very small amount (e.g., 0.0005 wt %). In other words, the above content range cannot be achieved by a typical method. The carbon nanotube structure has a configuration in which 2 to 5,000 single-walled carbon nanotube units are arranged and bonded to each other, so that the carbon nanotube structure is not broken even when the volume of the second negative electrode active material changes, and the length can be smoothly maintained. Therefore, the conductive network of the second negative electrode active material layer can be maintained, and the conductivity of the second negative electrode active material layer can be smoothly secured due to the high conductivity of the carbon nanotube structure. As a result, even if the content of the carbon nanotube structure in the second negative electrode active material layer is low, the input/output characteristics and life characteristics of the battery may be excellent.

On the other hand, in some cases, the single-walled carbon nanotube unit may be surface-treated by oxidation or nitridation to improve its affinity with the dispersant.

The second conductive material may further include at least one selected from the group consisting of fullerene, carbon black, carbon nanotube units, and graphene. In this case, the one-dimensional linear conductive network can become a two-dimensional or higher conductive network by the combined application of the material and the carbon nanotube structure, thereby improving the conductivity of the second negative electrode active material layer and improving the input/output characteristics and life characteristics of the battery.

The thickness of the second negative electrode active material layer may be 1 μm to 100 μm, specifically 5 μm to 90 μm, more specifically 10 μm to 80 μm. When this range is satisfied, the aforementioned conductive material and binder tilt phenomenon can be minimized. As a result, the adhesion of the negative electrode (the adhesion between the negative electrode active material layer and the current collector), the adhesion between the first negative electrode active material layer and the second negative electrode active material layer, and the electrical conductivity of the negative electrode can be significantly improved, and the input/output characteristics and life characteristics of the battery can be improved.

The thickness of the second negative electrode active material layer is preferably equal to or greater than the thickness of the first negative electrode active material layer. The ratio of the thickness of the first negative electrode active material layer to the thickness of the second negative electrode active material layer may be 10:90 to 50:50, specifically 20:80 to 50:50, and more specifically 25:75 to 50:50. If the above range is not satisfied, the effect of suppressing the migration of the conductive material and binder described above will be reduced, and the effect of improving the diffusion resistance due to the improvement of the porosity of the second negative electrode active material layer will be reduced. If the thickness of the first negative electrode active material layer is very thin outside the above range, the effect of suppressing the migration of the conductive material and binder described above will be reduced, and the effects of improving the negative electrode adhesion and the interface resistance will be weakened.

An interface exists between the first negative electrode active material layer and the second negative electrode active material layer. This can be confirmed by a cross-section of the manufactured negative electrode. In contrast, if the negative electrode active material layer is formed as a single layer structure (a single coating is performed using one negative electrode slurry) rather than a multi-layer structure, the interface is not observed.

The first negative electrode active material layer and the second negative electrode active material layer may each further include a binder, and the binder of the first negative electrode active material layer and the binder of the second negative electrode active material layer may be the same or different. The binder may be a general binder used in the technical field to ensure adhesion between the negative electrode active materials or between the negative electrode active material and the current collector, and the type of the binder is not particularly limited. Examples of the binder include vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), fluororubber, and various copolymers thereof, and one or more of these may be used alone or in combination.

The binder may be included in the first negative electrode active material layer (or the second negative electrode active material layer) in an amount of 10 wt % or less, preferably 0.1 wt % to 5 wt %. When the binder content is within this range, it is possible to realize excellent negative electrode adhesion while minimizing an increase in negative electrode resistance.

Method for Producing the Negative Electrode Next, a method for producing the negative electrode of the present invention will be described.

The method for manufacturing a negative electrode of the present invention includes the steps of preparing a first negative electrode slurry and a second negative electrode slurry; forming a first negative electrode active material layer on a negative electrode current collector using the first negative electrode slurry; and forming a second negative electrode active material layer on the first negative electrode active material layer using the second negative electrode slurry; the second negative electrode slurry includes a second negative electrode active material; and a second conductive material, the second negative electrode active material includes a silicon-based active material, the silicon-based active material includes SiO _x (0≦X<2), and the second conductive material includes a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded in a line; and the carbon nanotube structure may be included in the second negative electrode active material layer in an amount of 0.01 wt % to 1.0 wt %. The first negative electrode active material layer, the second negative electrode active material layer, the second negative electrode active material, the second conductive material, and the carbon nanotube structure are the same as those in the above-mentioned embodiments.

(1) Preparing First Negative Electrode Slurry and Second Negative Electrode Slurry The first negative electrode slurry may be prepared in the same manner as in a typical negative electrode slurry preparation method. For example, a mixture including a first negative electrode active material (same as the first negative electrode active material of the above-described embodiment), a first conductive material (same as the first conductive material of the above-described embodiment), and a solvent (which may further include a binder) is prepared, and the mixture is stirred to prepare the first negative electrode slurry.

However, if the first negative electrode slurry contains carbon nanotube structures, a carbon nanotube structure dispersion liquid described below must be prepared.

Examples of the solvent include water, dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP) and other amide-based polar organic solvents; methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, and octanol; glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, and hexylene glycol; and various solvents such as glycerin, trimethylolpropane, pentaerythritol, and sorbitol. Examples of the solvent include, but are not limited to, hydric alcohols; glycol ethers such as ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, and tetraethylene glycol monobutyl ether; ketones such as acetone, methyl ethyl ketone, methyl propyl ketone, and cyclopentanone; esters such as ethyl acetate, γ-butyrolactone, and ε-propiolactone, and any one or a mixture of two or more of these may be used. The solvent may be the same as or different from the dispersion medium used in the conductive material dispersion, and may be preferably water.

The second negative electrode slurry may be prepared by preparing a mixture containing the second negative electrode active material, a carbon nanotube structure dispersion liquid, and a solvent, and then stirring the mixture.

The carbon nanotube structure dispersion may be prepared as follows:

The preparation of the carbon nanotube structure dispersion may include the steps of: preparing a mixture containing a dispersion medium, a dispersant, and bundled single-walled carbon nanotubes (a combination or aggregate of single-walled carbon nanotube units) (S1-1); and applying a shear force to the mixture to disperse the bundled single-walled carbon nanotubes and form a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bound together in a line (S1-2).

In step S1-1, the mixture may be prepared by adding bundled single-walled carbon nanotubes and a dispersant to a dispersion medium. The bundled single-walled carbon nanotubes are formed by combining the above-mentioned single-walled carbon nanotube units into a bundle, and typically contain 2 or more, substantially 500 or more, for example, 5,000 or more single-walled carbon nanotube units.

The specific surface area of the bundled single-walled carbon nanotubes may be 500 ^{m 2} /g to 1,200 ^{m 2} /g, specifically, 500 m ² /g to 1,000 ^{m 2} /g. If the range is satisfied, the large specific surface area can smoothly secure a conductive path in the second negative electrode active material layer, and therefore, even if the conductive material content is very small, the conductivity in the second negative electrode active material layer can be maximized. In addition, in order to strengthen the adhesive strength between the first negative electrode active material layer and the second negative electrode active material layer, the specific surface area is preferably 500 m ² /g to 800 ^{m 2} /g.

The bundled single-walled carbon nanotubes may be contained in the mixture at 0.1% to 1.0% by weight, specifically 0.2% to 0.5% by weight. When this range is satisfied, the bundled single-walled carbon nanotubes are dispersed at an appropriate level, a carbon nanotube structure at an appropriate level can be formed, and dispersion stability can be improved.

Examples of the dispersion medium include water, amide polar organic solvents such as dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP); alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, and octanol; glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, and hexylene glycol; and various solvents such as glycerin, trimethylolpropane, pentaerythritol, and sorbitol. Examples of the dispersing medium include, but are not limited to, glycol ethers such as ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, and tetraethylene glycol monobutyl ether; ketones such as acetone, methyl ethyl ketone, methyl propyl ketone, and cyclopentanone; esters such as ethyl acetate, γ-butyrolactone, and ε-propiolactone, and any one or a mixture of two or more of these may be used. More specifically, the dispersing medium may be the same as or different from the solvent for producing the negative electrode slurry, and may be preferably water.

The dispersant may include at least one of hydrogenated nitrile butadiene rubber, polyvinylidene fluoride, polystyrene, polyvinylpyrrolidone, polyvinyl alcohol, pyrene butyric acid, pyrene sulfonic acid, tannic acid, pyrene methylamine, sodium dodecyl sulfate, and carboxymethyl cellulose, and may be specifically carboxymethyl cellulose, polyvinylidene fluoride, polyvinylpyrrolidone, or hydrogenated nitrile butadiene rubber.

In the carbon nanotube structure dispersion, the weight ratio of the bundled carbon nanotubes to the dispersant may be 1:0.1 to 1:10, specifically 1:1 to 1:10. When this range is satisfied, the bundled single-walled carbon nanotubes are dispersed to an appropriate level, a carbon nanotube structure of an appropriate level can be formed, and dispersion stability can be improved.

The solid content in the mixture may be 0.1% by weight to 20% by weight, specifically 1% by weight to 10% by weight. When this range is satisfied, the bundled single-walled carbon nanotubes are dispersed at an appropriate level, a carbon nanotube structure of an appropriate level can be formed, and dispersion stability can be improved. In addition, the second negative electrode slurry (slurry for manufacturing the second negative electrode active material layer) can have viscosity and elasticity suitable for forming the second negative electrode active material layer, which also contributes to increasing the solid content of the second negative electrode slurry.

In step S1-2, the process of dispersing the bundle-type carbon nanotubes in the mixture may be performed using a mixing device such as a homogenizer, a bead mill, a ball mill, a basket mill, an attrition mill, a universal mixer, a clear mixer, a spike mill, a TK mixer, or an ultrasonic dispersion (sonification) device. Among them, the bead mill method is preferred since it can precisely control the diameter size of the carbon nanotube structure, can achieve uniform distribution of the carbon nanotube structure, and has an advantage in terms of cost.

The bead mill method may be as follows: The mixture is placed in a container containing beads, etc., and the container is rotated to disperse the bundled single-walled carbon nanotubes.

Here, the conditions under which the bead mill method is carried out are as follows:

The average particle size of the beads may be 0.5 mm to 1.5 mm, specifically 0.5 mm to 1.0 mm. If this range is met, the carbon nanotube structure will not be cut during the dispersion process, the diameter size can be appropriately controlled, and a dispersion solution with a uniform composition can be produced.

The rotation speed of the container may be 500 RPM to 10,000 RPM, specifically 2,000 RPM to 6,000 RPM. If this range is met, the carbon nanotube structures will not be cut during the dispersion process, the diameter size can be appropriately controlled, and a dispersion solution with a uniform composition can be produced.

The time for which the bead mill is performed may be 0.5 hours to 2 hours, specifically 0.5 hours to 1.5 hours, and more specifically 0.8 hours to 1 hour. If this range is satisfied, the carbon nanotube structures are not cut during the dispersion process, and the diameter size can be appropriately controlled, making it possible to produce a dispersion solution with a uniform composition. The bead mill performance time means the total time the bead mill is applied, so for example, if the bead mill is performed several times, it means the total time over those several times.

The bead mill conditions are for dispersing the bundled single-walled carbon nanotubes to an appropriate level, specifically excluding the case where the bundled single-walled carbon nanotubes are completely dispersed into a single single-walled carbon nanotube. In other words, the bead mill conditions are for dispersing the bundled single-walled carbon nanotubes to an appropriate level, and forming a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded to each other in a line in the produced carbon nanotube structure dispersion liquid. This can only be achieved when the composition of the mixture liquid, the conditions of the dispersion process (e.g., the bead mill process), etc. are strictly controlled.

By the above process, a carbon nanotube structure dispersion can be formed.

The negative electrode slurries (first negative electrode slurry and second negative electrode slurry) may further include a binder as necessary. Here, the binder may be the binder of the above-mentioned embodiment.

(2) Forming a first negative electrode active material layer on a negative electrode current collector using the first negative electrode slurry and forming a second negative electrode active material layer on the first negative electrode active material layer using the second negative electrode slurry. Next, a first negative electrode active material layer is formed using the first negative electrode slurry prepared as described above. Specifically, the first negative electrode active material layer may be formed by applying the first negative electrode slurry on a negative electrode current collector and then drying it, or by applying the first negative electrode slurry on a separate support and then peeling it off from the support to obtain a film and laminating it on the negative electrode current collector. If necessary, a rolling process may be further performed after forming the first negative electrode active material layer using the above method. In this case, the drying and rolling may be performed under appropriate conditions in consideration of the physical properties of the negative electrode to be finally manufactured, and are not particularly limited.

Then, the second negative electrode active material layer is formed using the second negative electrode slurry prepared as described above. Specifically, the second negative electrode active material layer may be formed by applying the second negative electrode slurry onto the first negative electrode active material layer and then drying it, or by applying the second negative electrode slurry onto a separate support and then peeling it off from the support to obtain a film, which is then laminated onto the first negative electrode active material layer. If necessary, after forming the second negative electrode active material layer by the above method, a rolling process may be further performed. In this case, the drying and rolling may be performed under appropriate conditions in consideration of the physical properties of the negative electrode to be finally manufactured, and are not particularly limited.

Secondary Battery Next, a secondary battery according to another embodiment of the present invention will be described.

A secondary battery according to another embodiment of the present invention may include the negative electrode of the above-described embodiment.

Specifically, the secondary battery may include the negative electrode, the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, and the negative electrode is the same as the negative electrode in the above-mentioned embodiment. The negative electrode has been described above, so a detailed description thereof will be omitted.

The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and including the positive electrode active material.

In the positive electrode, the positive electrode current collector is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used. The positive electrode current collector may usually have a thickness of 3 μm to 500 μm, and the adhesive strength of the positive electrode active material can be increased by forming fine irregularities on the surface of the current collector. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO ₂ ) or lithium nickel oxide (LiNiO ₂ ), or a compound substituted with one or more transition metals; a lithium iron oxide such as LiFe ₃ O ₄ ; a lithium manganese oxide such as LiMnO 3 , LiMn ₂ O 3 , or LiMnO 2 having the chemical formula Li _1+c1 Mn _2-c1 O ₄ (0≦c1≦0.33); a lithium copper oxide (Li ₂ CuO ₂ ); a vanadium oxide such as LiV _{3 O 8} , V ₂ O ₅ , or Cu ₂ V ₂ O ₇ ; or a _lithium manganese oxide such as LiNi 1-c2 M c2 O 2 having the _chemical formula LiNi _1-c2 M _c2 O _{2 .} Examples of the lithium-nickel oxide include, but are not limited to, a Ni-site type lithium-nickel oxide represented by the formula LiMn _2-c3 M _c3 O ₂ (wherein M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, and 0.01≦c3≦0.1); a lithium-manganese composite oxide represented by the formula LiMn 2-c3 M c3 O _{2 (wherein M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, and 0.01≦c3≦0.1) or Li 2 Mn 3 MO 8 ( wherein} M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); and LiMn ₂ O ₄ in which a part of Li in the formula is replaced with an alkaline earth metal ion. The positive electrode may be Li-metal.

The positive electrode active material layer may contain a positive electrode conductive material and a positive electrode binder in addition to the positive electrode active material described above.

In this case, the positive electrode conductive material is used to impart conductivity to the electrode, and can be used without any particular restrictions as long as it has electronic conductivity without causing a chemical change in the battery that is constructed. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fibers; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and the like. One or a mixture of two or more of these may be used.

In addition, the positive electrode binder plays a role in improving the adhesion between the positive electrode active material particles and the adhesive strength between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, and various copolymers thereof, and one or more of these may be used alone or in combination.

The separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. Any separator that is generally used in secondary batteries can be used without any particular restrictions. In particular, a separator that has low resistance to the ion movement of the electrolyte and has excellent moisture absorption capacity for the electrolyte solution is preferable. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer, or a laminate structure of two or more layers thereof may be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high-melting point glass fiber, polyethylene terephthalate fiber, etc. may be used. In addition, a coated separator containing a ceramic component or a polymeric substance may be used to ensure heat resistance or mechanical strength, and may be selectively used in a single-layer or multi-layer structure.

The electrolyte may be, but is not limited to, an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a molten inorganic electrolyte that can be used in the manufacture of a lithium secondary battery.

Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

As the non-aqueous organic solvent, for example, aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, and ethyl propionate may be used.

In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates among the carbonate-based organic solvents, are highly viscous organic solvents with high dielectric constants that dissociate lithium salts well, and are therefore preferably used. When such cyclic carbonates are mixed with low-viscosity, low-dielectric-constant linear carbonates such as dimethyl carbonate and diethyl carbonate in an appropriate ratio, an electrolyte with high electrical conductivity can be produced, and these can be used more preferably.

The metal salt may be a lithium salt, which is a substance that is easily dissolved in the non-aqueous electrolyte. For example, anions of the lithium salt include F ^- , Cl ^- , I ^- , NO ₃ ^- , N(CN) ₂ ^- , BF ₄ ^- , ClO ₄ -, PF ₆ -, (CF ₃ ) ₂ PF ₄ ^- , (CF 3 ) ₃ PF ₃ ^- , (CF ₃ ) ₄ PF ₂ ^- , (CF ₃ ) ₅ PF ^- , (CF ₃ ) ₆ P ^- , CF ₃ SO ₃ ^- , CF ₃ CF ₂ ^SO _{3 -} ^, (CF ₃ SO ₂ ) ₂ N ^- , (FSO ₂ ) ₂ N ^- , CF _{3 One or more selected} from _{the group consisting of CF2(CF3)2CO-, (CF3SO2)2CH-, (SF5)3C-} ^{, (} _{CF3SO2 ) 3C-} ^, _CF3 ⁽ _CF2 ⁾ _7SO3- ^, _{CF3CO2- ,} ^CH3CO2- _{, SCN- , and ( CF3CF2SO2} ⁾ _{2N- may be used} ^.

In addition to the components of the electrolyte, the electrolyte may further contain one or more additives such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing the decrease in battery capacity, and improving the discharge capacity of the battery.

According to yet another embodiment of the present invention, there is provided a battery module including the secondary battery as a unit cell, and a battery pack including the same. The battery module and the battery pack include the secondary battery having high capacity, high-speed characteristics, and cycle characteristics, and may be used as a power source for a medium- to large-sized device selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

The present invention will be explained in more detail below with specific examples.

Preparation Example 1: Preparation of Carbon Black Dispersion 0.4 parts by weight of carbon black (Imerys, Super C65) having an average particle size of 35 nm and 0.6 parts by weight of carboxymethyl cellulose (weight average molecular weight: 100,000 g/mol, degree of substitution: 1.0) were mixed with 99.0 parts by weight of water as a dispersion medium to prepare a mixture having a solid content of 1.0% by weight. The mixture was stirred using a beads-mill to disperse the carbon black in the solvent to prepare a carbon black dispersion. In this case, the particle size of the beads was 1 mm, the rotation speed of the stirring vessel containing the beads was 3,000 RPM, and the stirring was performed for 60 minutes.

In the carbon black dispersion, the carbon black was 0.4% by weight and the carboxymethyl cellulose was 0.6% by weight.

Preparation Example 2: Preparation of carbon nanotube structure dispersion 0.4 parts by weight of bundled carbon nanotubes (specific surface area: 650 ^m2 /g) consisting of single-walled carbon nanotube units having an average diameter of 1.5 nm and an average length of 5 μm or more, and 0.6 parts by weight of carboxymethyl cellulose (weight average molecular weight: 100,000 g/mol, degree of substitution: 1.0) were mixed with 99.0 parts by weight of water as a dispersion medium to prepare a mixture with a solid content of 1.0 wt%.

The mixture was stirred using a bead mill to disperse the bundled single-walled carbon nanotubes in the solvent, producing a carbon nanotube structure dispersion. In this case, the beads had a particle size of 1 mm, the rotation speed of the stirring vessel containing the beads was 3,000 RPM, and the stirring was performed for 60 minutes. The carbon nanotube structure dispersion contained carbon nanotube structures in which 2 to 5,000 single-walled carbon nanotube units were bound together in a line.

In the carbon nanotube structure dispersion, the carbon nanotube structure was 0.4% by weight and the carboxymethyl cellulose was 0.6% by weight.

Production Example 3: Production of carbon nanotube structure dispersion A carbon nanotube structure dispersion was produced in the same manner as in Production Example 2, except that the weight average molecular weight of carboxymethyl cellulose was changed to 400,000 g/mol (substitution degree: 1.0). In the dispersion, the carbon nanotube structure was 0.4 wt % and the carboxymethyl cellulose was 0.6 wt %.

Preparation Example 4: Preparation of single-walled carbon nanotube unit dispersion 0.2 parts by weight of bundled carbon nanotubes (specific surface area: 650 ^m2 /g) consisting of single-walled carbon nanotube units having an average diameter of 1.5 nm and an average length of 5 μm or more, and 1.2 parts by weight of carboxymethyl cellulose (weight average molecular weight: 100,000 g/mol, degree of substitution: 1.0) were mixed with 98.6 parts by weight of water as a dispersion medium to prepare a mixture with a solid content of 1.4% by weight.

The mixture was stirred in a bead mill to disperse the bundled single-walled carbon nanotubes in the solvent, producing a single-walled carbon nanotube unit dispersion. In this case, the particle size of the beads was 1 mm, and the rotation speed of the stirring vessel containing the beads was 3,000 RPM. Stirring was performed for a total of four cycles (with 60 minutes of natural cooling between each cycle) under the above conditions, with 60 minutes of stirring being one cycle. In this way, a single-walled carbon nanotube unit dispersion was produced. In the dispersion, the bundled single-walled carbon nanotubes were completely dispersed, and only single-walled carbon nanotube units were present as individual pieces, and the above-mentioned carbon nanotube structure was not detected. In addition, the single-walled carbon nanotube units were 0.2 wt % and the carboxymethyl cellulose was 1.2 wt % in the single-walled carbon nanotube unit dispersion.

Preparation Example 5: Preparation of multi-walled carbon nanotube unit dispersion 4.0 parts by weight of bundled carbon nanotubes (specific surface area: 185 ^m2 /g) consisting of multi-walled carbon nanotube units having an average diameter of 10 nm and an average length of 1 μm, and 0.6 parts by weight of carboxymethyl cellulose (weight average molecular weight: 100,000 g/mol, degree of substitution: 1.0) were mixed with 95.4 parts by weight of water as a dispersion medium to prepare a mixture having a solid content of 4.6% by weight.

The mixture was dispersed in a spike mill filled with 80% beads of 0.65 mm in size, and discharged at a discharge rate of 2 kg/min. This process was repeated twice to completely disperse the bundled multi-walled carbon nanotubes, producing a multi-walled carbon nanotube unit dispersion. In the dispersion, the multi-walled carbon nanotube units (average diameter: 10 nm) were 4.0 wt % and the carboxymethyl cellulose was 0.6 wt %.

Examples and Comparative Examples Example 1: Preparation of Negative Electrode (1) Formation of First Negative Electrode Active Material Layer The carbon black dispersion of Preparation Example 1, artificial graphite having an average particle size ( _D50 ) of 21 μm, and binders styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) (weight average molecular weight: 100,000 g/mol, degree of substitution: 1.0) were mixed with water to prepare a first negative electrode slurry. The negative electrode slurry was applied to a negative electrode current collector (copper (Cu) metal thin film) having a thickness of 20 μm, and then dried at 130° C. and rolled to form a first negative electrode active material layer (thickness: 50 μm).

In the first negative electrode active material layer, the artificial graphite was contained at 94.80 wt%, the SBR at 3.5 wt%, the CMC at 1.2 wt%, and the carbon black at 0.50 wt%.

(2) Formation of Second Negative Electrode Active Material Layer A second negative electrode slurry was prepared by mixing the carbon nanotube structure dispersion of Preparation Example 2, a negative electrode active material (weight ratio of artificial graphite having an average particle size ( _D50 ) of 21 μm:SiO having an average particle size ( _D50 ) of 6.6 μm=94:6), and a binder of SBR and carboxymethyl cellulose (CMC) (weight average molecular weight: 100,000 g/mol, degree of substitution: 1.0) with water. The second negative electrode slurry was applied onto the first negative electrode active material layer, and then dried at 130° C. and rolled to form a second negative electrode active material layer (thickness: 50 μm).

In the second negative electrode active material layer, the negative electrode active material is contained in a total amount of 96.25 wt%, the SBR is contained in a total amount of 2.5 wt%, the CMC is contained in a total amount of 1.2 wt%, and the carbon nanotube structure is contained in a total amount of 0.05 wt%.

Example 2: Preparation of negative electrode A negative electrode was prepared in the same manner as in Example 1, except that the carbon nanotube structure dispersion liquid of Preparation Example 3 was used instead of the carbon nanotube structure dispersion liquid of Preparation Example 2 when forming the second negative electrode active material layer in Example 1.

Example 3: Preparation of negative electrode A negative electrode was prepared in the same manner as in Example 1, except that the multi-walled carbon nanotube unit dispersion liquid of Preparation Example 5 was used instead of the carbon black dispersion liquid of Preparation Example 1 when forming a first negative electrode slurry in Example 1.

Example 4: Preparation of Negative Electrode A negative electrode was prepared in the same manner as in Example 1, except that the carbon nanotube structure dispersion of Preparation Example 2 and the multi-walled carbon nanotube unit dispersion of Preparation Example 5 were used instead of the carbon black dispersion of Preparation Example 1 when forming the first negative electrode slurry in Example 1. In the prepared first negative electrode active material layer, the weight ratio of the carbon nanotube structure to the multi-walled carbon nanotubes was 10:90. In the first negative electrode active material layer, the artificial graphite was 94.80 wt%, the SBR was 3.5 wt%, the CMC was 1.2 wt%, the carbon nanotube structure was 0.05 wt%, and the multi-walled carbon nanotube units were 0.45 wt%.

Example 5: Preparation of negative electrode A negative electrode was prepared in the same manner as in Example 1, except that the carbon nanotube structure dispersion liquid of Preparation Example 2 was used instead of the carbon black dispersion liquid of Preparation Example 1 when forming the first negative electrode slurry in Example 1.

The first negative electrode active material layer contains 95.25 wt % of the artificial graphite, 3.5 wt % of the SBR, 1.2 wt % of the CMC, and 0.05 wt % of the carbon nanotube structure.

Comparative Example 1: Preparation of Negative Electrode A negative electrode slurry was prepared by mixing the carbon nanotube structure dispersion of Preparation Example 2, a negative electrode active material (artificial graphite having an average particle size ( _D50 ) of 21 μm:SiO having an average particle size ( _D50 ) of 6.6 μm in a weight ratio of 94:6), and a binder, SBR and carboxymethyl cellulose (CMC) (weight average molecular weight: 100,000 g/mol, degree of substitution: 1.0), with water. The negative electrode slurry was applied to a negative electrode current collector (copper (Cu) metal thin film) having a thickness of 20 μm, and then dried at 130° C. and rolled to form a negative electrode active material layer (thickness: 100 μm).

In the negative electrode active material layer, the negative electrode active material is contained in a total of 95.75 wt%, the SBR is contained in a total of 3.0 wt%, the CMC is contained in a total of 1.2 wt%, and the carbon nanotube structure is contained in a total of 0.05 wt%.

Comparative Example 2: Preparation of Negative Electrode A negative electrode was prepared in the same manner as in Example 1, except that the single-walled carbon nanotube unit dispersion liquid of Preparation Example 4 was used instead of the carbon nanotube structure of Preparation Example 2 when forming the second negative electrode active material layer of Example 1.

In the second negative electrode active material layer, the negative electrode active material was included in a total amount of 96.25 wt % (weight ratio of artificial graphite having an average particle size ( _D50 ) of 21 μm: SiO having an average particle size ( _D50 ) of 6.6 μm=94:6), the SBR was included in a total amount of 2.5 wt %, the CMC was included in a total amount of 1.2 wt %, and the single-walled carbon nanotube units were included in a total amount of 0.05 wt %.

Comparative Example 3: Preparation of Negative Electrode A negative electrode was prepared in the same manner as in Example 1, except that the multi-walled carbon nanotube unit dispersion liquid of Preparation Example 5 was used instead of the carbon nanotube structure of Preparation Example 2 when forming the second negative electrode active material layer of Example 1.

In the second negative electrode active material layer, the negative electrode active material was included in a total amount of 95.80 wt % (weight ratio of artificial graphite having an average particle size ( _D50 ) of 21 μm: SiO having an average particle size ( _D50 ) of 6.6 μm=94:6), the SBR was included in a total amount of 2.5 wt %, the CMC was included in a total amount of 1.2 wt %, and the multi-walled carbon nanotube units were included in a total amount of 0.5 wt %.

In Examples 1, 3 to 5 and Comparative Example 1, the carbon nanotube structures had an average diameter of 10 nm and an average length of 8.2 μm. In Example 2, the carbon nanotube structures had an average diameter of 100 nm and an average length of 15.6 μm. In Comparative Example 2, the single-walled carbon nanotube units had an average diameter of 1.6 nm and an average length of 1.8 μm.

In the negative electrodes of Examples 3 and 4 and Comparative Example 3, the average diameter of the multi-walled carbon nanotube units was 10.8 nm and the average length was 1.3 μm.

The average diameter and average length correspond to the average values of the top 100 carbon nanotube structures (or multi-wall carbon nanotube units or single-wall carbon nanotube units) with the largest diameter (or length) and the bottom 100 carbon nanotube structures (or multi-wall carbon nanotube units or single-wall carbon nanotube units) when the manufactured negative electrode is observed through a TEM.

Experimental Example 1: Observation of negative electrode (1) Binder distribution FIG. 1 is a photograph analyzing the binder distribution of the negative electrode of Comparative Example 1 and the negative electrode of Example 1. The analysis was performed by an osmium oxide (OsO ₄ ) dyeing method. Specifically, the negative electrode was dyed by exposing/standing in an osmium oxide fume atmosphere for 3 days in a closed special chamber prepared in a glove box, and the atmosphere was replaced with Ar. After 1 day, the cross section of the negative electrode cut by ion milling was analyzed by image mapping using SEM-EDX.

Figure 1 shows analytical photographs of Comparative Example 1 (left), in which a single layer of the negative electrode active material layer is formed, and Example 1 (right), in which two layers of first and second negative electrode active material layers are formed. Looking at the photograph on the right, it can be seen that in Example 1, the first and second negative electrode active material layers have different compositions, and the binder distribution ratios for the first and second negative electrode active material layers are also different. In particular, when the first and second negative electrode active material layers are formed as in Example 1, it can be seen that SiO is located only in the second negative electrode active material layer, and the binder amount in the first negative electrode active material layer is greater. It can also be seen that the binder distribution changes in the thickness direction of the negative electrode active material layer.

(2) Confirmation of the presence of carbon nanotube structures Fig. 2 is an SEM photograph of the second negative electrode active material layer of the negative electrode of Example 1, and Fig. 3 is an SEM photograph of the second negative electrode active material layer of the negative electrode of Example 2. Fig. 4 is an SEM photograph of the second negative electrode active material layer of the negative electrode of Comparative Example 3. Fig. 6 is an SEM photograph of the second negative electrode active material layer of the negative electrode of Comparative Example 2.

Referring to Figures 2 and 3, it can be seen that there is a long rope-like carbon nanotube structure in which multiple single-walled carbon nanotube units are arranged side by side and connected to each other. In particular, in the negative electrode of Example 1 in Figure 2, hydrogenated nitrile butadiene rubber with a relatively low weight average molecular weight was used, so it can be seen that a carbon nanotube structure with an average diameter of about 10 nm was formed, and in the case of Figure 3, it can be seen that a carbon nanotube structure with an average diameter of about 100 nm was formed. In both Figures 2 and 3, it can be seen that the carbon nanotube structures form a network structure.

On the other hand, in Figure 4, only short multi-walled carbon nanotube units were visible, and no carbon nanotube structure was observed. Also, in Figure 6, only single-walled carbon nanotube units existing as individual units were visible, and no carbon nanotube structure was observed.

(3) Confirmation of the network by the second conductive material Figure 5 is an SEM image of the second negative electrode active material layer of the negative electrode of Example 1. Referring to (A) of Figure 5, it can be seen that the carbon nanotube structure can suppress cracks in SiO. Referring to (B) of Figure 5, it can be seen that the carbon nanotube structure keeps the conductive network from being cut on the SiO where cracks have occurred. Referring to (C) of Figure 5, it can be seen that the conductive network is formed long due to the long length of the carbon nanotube structure.

Experimental Example 2: Evaluation of negative electrode adhesion The negative electrode adhesion (adhesion between the negative electrode active material layer and the current collector) was measured under dry conditions. Specifically, double-sided tape was attached to a glass slide, and a negative electrode punched to 20 mm x 180 mm was placed on the tape and rolled back and forth 10 times with a 2 kg roller to adhere the negative electrode. The force required to peel the negative electrode from the glass slide was measured by pulling the negative electrode at 200 mm/min using a UTM (TA) device. The measurement angle between the glass slide and the negative electrode was 90°. The measurement results are shown in Table 2 below.

Experimental Example 3: Evaluation of Adhesion Strength of First Negative Electrode Active Material and Second Negative Electrode Active Material For Examples 1 to 4 and Comparative Examples 1 to 3, the shear strength (N/mm ² ) was measured by the following method. The measurement was performed using a SAICAS (Surface and Interfacial Characterization Analysis System) device (SAICAS EN-EX, Daipla Wintes Japan). Specifically, a micro-sized diamond blade was used to cut each negative electrode in a tilted state from the surface toward the inside, and the shear strength was measured at the interface between the first negative electrode active material layer and the second negative electrode active material layer according to the force applied to the blade, and the results are shown in Table 2.

Experimental Example 4: Evaluation of discharge capacity and capacity retention rate according to C-Rate Using the negative electrodes of Examples 1 to 5 and Comparative Examples 1 to 3, batteries were fabricated as follows.

Li _{[ Ni0.6Mn0.2Co0.2} ] _O2 was used as _the positive electrode active material. The positive electrode active material, carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed in a weight ratio of 94:4:2 in N-methyl-2-pyrrolidone as a solvent to prepare a positive electrode slurry.

The prepared positive electrode slurry was applied to a 15 μm-thick aluminum metal thin film, which was a positive electrode current collector, and dried. At this time, the temperature of the circulating air was 110°C. Next, it was rolled and dried in a vacuum oven at 130°C for 2 hours to form a positive electrode active material layer.

Each of the negative electrodes of Examples 1 to 5 and Comparative Examples 1 to 3, the positive electrodes prepared above, and a porous polyethylene separator were stacked together, and an electrolyte (ethylene carbonate (EC)/ethyl methyl carbonate (EMC)=1/2 (volume ratio), lithium hexafluorophosphate (LiPF ₆ 1 mol)) was injected into the assembled battery to prepare a lithium secondary battery.

1) Evaluation of Discharge Capacity by C-Rate The charge C-Rate was fixed at 0.2 C, and the discharge C-Rate was increased from 0.2 C to 2.0 C. The 0.2 C discharge capacity and the 2.0 C discharge capacity (%) of each lithium secondary battery were measured and shown in Table 2.

2) Evaluation of Capacity Retention Rate (Life Characteristics) Each lithium secondary battery was charged and discharged under the following conditions.
Each lithium secondary battery was charged/discharged at 0.33 C/0.33 C in the voltage range of 4.25 V to 2.8 V at 45° C. for a total of 100 cycles. Thereafter, the discharge capacity after 100 cycles (capacity retention rate) was evaluated based on the discharge capacity after one cycle being 100%. The results are shown in Table 2.

Referring to Table 2, it can be seen that when the second negative electrode active material layer contains a carbon nanotube structure, the input/output characteristics and life characteristics of the battery are improved, and the negative electrode adhesion and the adhesion between the first negative electrode active material layer and the second negative electrode active material layer can be improved.

Claims (12)

a negative electrode current collector; a first negative electrode active material layer disposed on the negative electrode current collector; and a second negative electrode active material layer disposed on the first negative electrode active material layer,
the first negative electrode active material layer includes a first negative electrode active material,
the first negative electrode active material is a carbon-based active material;
the second negative electrode active material layer includes a second negative electrode active material and a second conductive material,
the second negative electrode active material includes a silicon-based active material;
The silicon-based active material contains SiO _x (0≦X<2),
The second conductive material includes a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded in a line,
The carbon nanotube structure is included in the second negative electrode active material layer in an amount of 0.01 wt % to 1.0 wt %;
The first negative electrode active material layer further includes a first conductive material,
The first conductive material includes at least one selected from the group consisting of the carbon nanotube structure and a multi-walled carbon nanotube unit . The negative electrode according to claim 1, wherein the carbon nanotube structures in the second negative electrode active material layer are interconnected to form a network structure. In the carbon nanotube structure,
The negative electrode of claim 1 , wherein the single-walled carbon nanotube units are bonded together with their major axes aligned parallel to each other. The negative electrode according to claim 1, wherein the average length of the carbon nanotube structure is 1 μm to 500 μm. The negative electrode according to claim 1, wherein the average length of the carbon nanotube structure is 2 μm to 50 μm. The negative electrode according to claim 1, wherein the average diameter of the carbon nanotube structure is 2 nm to 500 nm. The negative electrode according to claim 1, wherein the average diameter of the carbon nanotube structure is 5 nm to 200 nm. The negative electrode of claim 1, wherein the average diameter of the single-walled carbon nanotube units in the carbon nanotube structure is 0.5 nm to 10 nm. The negative electrode according to claim 1, wherein the carbon nanotube structure is a carbon nanotube structure in which 2 to 50 single-walled carbon nanotube units are bonded to each other. The negative electrode according to claim 1, wherein the thickness of the first negative electrode active material layer is 1 μm to 100 μm. The negative electrode according to claim 1, wherein the thickness of the second negative electrode active material layer is 1 μm to 100 μm. A secondary battery comprising the negative electrode according to claim 1.

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